Pretty bad argument. If it’s not defined by POSIX, it’s not POSIX compatible if you rely on a specific behavior.
If you only use defined behavior and it works, it is compatible.
It’s like saying C99 isn’t a compiler. True, but you can still write C99 code, right?
> C99 isn’t a compiler.
Sure, but the pojt here is that if we say "Write in X" we generally understand it to mean "Treat X like a standard and don't get too colloquial with the stylings."
Pedantry is worthwhile, but it can be a diminishing returns game.